Cutting planes within a transparent material are generated, particularly, in laser-surgical methods, and especially in ophthalmic surgery.
Thereby, the treatment laser radiation within the tissue, i.e. beneath the tissue surface, is focused in such a way that optical breakthroughs in the tissue are formed.
Thereby, several processes initiated by the laser radiation occur in a time sequence in the tissue. If the power density of the radiation exceeds a threshold value, an optical breakthrough will occur, generating a plasma bubble in the material. After the optical breakthrough has been generated, said plasma bubble grows due to expanding gases. If the optical breakthrough is not maintained, the gas generated in the plasma bubble will be absorbed by the surrounding material and the bubble disappears again. However, this process takes very much longer than the forming of the bubble itself. If a plasma is generated at a material boundary, which may also be located within a material structure, material will be removed from said boundary.
This boundary phenomenon is then referred to as photoablation. In connection with a plasma bubble which separates previously connected material layers, the term photodisruption is usually applied. For the sake of simplicity, all such processes herein are collectively termed optical breakthrough, i.e. said term includes not only the actual optical breakthrough, but also the effects resulting therefrom in the material.
For high accuracy of a laser-surgical method, it is imperative to ensure high localization of the effect of the laser beams and preferably avoid collateral damage to adjacent tissue. It is therefore common in prior art to apply the laser radiation in pulsed form, so that the threshold value required for the triggering of an optical breakthrough is exceeded only during the individual pulses for the power density. In this regard, U.S. Pat. No. 5,984,916 clearly shows that the spatial extent of the optical breakthrough (in this case, the generated interaction) strongly depends on the pulse duration.
Therefore, high focusing of the laser beam in combination with very short pulses allows for placing of the optical breakthrough in a material with pinpoint accuracy.
The use of pulsed laser radiation has recently become established practice in ophthalmology, particularly for laser-surgical correction of defective vision.
Defective vision of the eye often results from the fact that the refractive properties of the cornea and the lens do not effect optimal focusing on the retina.
Aforementioned U.S. Pat. No. 5,984,916 as well as U.S. Pat. No. 6,110,166 describe generic methods of producing cuts by means of a suitable generation of optical breakthroughs, so that, ultimately, the refractive properties of the cornea are specifically influenced. A multitude of optical breakthroughs are sequentially arranged in such a way that a lens-shaped partial volume is isolated within the cornea of the eye. The lens-shaped partial volume, which is separated from the remaining corneal tissue, is then removed from the cornea through a laterally opening cut. The shape of the partial volume is selected in such a way that, after removal, the shape and, thus, the refractive properties of the cornea are modified in such a way that the desired correction of the visual defect is effected. The cutting planes required hereto are curved, which makes a three-dimensional shifting of the focus necessary. Therefore, a two-dimensional deflection of the laser radiation is combined with simultaneous shifting of the focus in a third spatial direction.
A further application of producing a cut by means of pulsed laser radiation in the cornea is the generation of so-called flaps, i.e., a cut which partially severs a small slice of the cornea in such a way that it can be folded back, making the underlying tissue of an ablation accessible by means of an excimer laser. Hereby, the desired cornea profile is produced through the ablation and the flap returned to its original position after treatment.
The two-dimensional deflection of the laser radiation is, similar to the focus shift, equally crucial for the accuracy with which the cutting plane can be produced. For the two-dimensional beam guidance, i.e., for the movement of the focus essentially in the plane of the cut, two strategies have been applies thus far. In DE 103 34 110, it is suggested to move the focus essentially in a closed path and to increase and/or decrease the path radius after each rotation by the value which approximately corresponds with the diameter of the focus. The generation of the cutting plane can therefore be effected alternatively from the inside out or from the outside in. The guidance of the focus on a spiral path is also described. In EP 1 591 087, it is suggested to start the cut on the outside and guide it inwards by means of a spiral-shaped path with steadily decreasing radius.
In the field, it has become apparent that both strategies do not lead to optimal results. While the vision of the patient immediately worsens with a cut from the inside out due to bubbles forming in the center, making it impossible for the patient to target the usually existing fixation marker and causing interfering eye movements, there are also problems with the cut from the outside in with regard to correctly applying the necessary opening cut on the cutting plane since the focus must once again be positioned in the peripheral area, where there are still bubbles from the previous cutting direction in the tissue, which make the exact positioning of the focus difficult.